Plant Successions and Paleoecological Reconstruction: Case Studies from the Neogene of the Northern Intermountain Region

The close relationship between climate and the nature of vegetation developed on a landscape supports the widespread use of fossil plant assemblages to reconstruct late Mesozoic and Cenozoic paleoclimates. Irrespective of the specific analytic methodologies, it is generally accepted that sufficiently diverse fossil floras are, in fact, exemplars of the prevailing climatic regime. In areas where large-scale vegetation disturbance is pervasive, such assumptions are simplistic at best. The Neogene of the Northern Intermountain region is characterized by fossil plant assemblages whose preservation was inextricably linked to widespread volcanism. Three exemplary assemblages (Succor Creek, Trapper Creek, and Stinking Water) suggest that volcanism and holocaustic fires appear to trigger similar distinctive patterns of ecological recovery with distinctive seral stages drawn from subsets of the regional biological inventory. While early recovery stages stages, dominated by non-woody weed analogs, tend to be ephemeral and not well-documented in the megafossil record, this is not the case with later seral stages dominated by woody vegetation. These later stages of vegetation recovery are typically quite different in composition/aspect compared to minimal-disturbance communities. Given the coupling between volcanic activity and plant fossil occurrences, such seral stages commonly dominate fossil plant assemblages. If such "floras" are inadvertently used for paleoclimatic reconstruction, significant interpretive errors are likely. The result is a high level of ecological "noise" that is interpreted as paleoclimatic variability. Understanding the disturbance-driven vegetation dynamics in any region is essential to identifying those fossil plant assemblages best suited to paleoclimatic reconstruction.